You can't actually "beat" Sephiroth, you just survive long enough to go to a cut scene.
I recommend using a phenix down before the battle, equiping Costly Punch, Jump, and Cure or Curaga. The First part of the fight is easy, but the second half is very difficult.
Sadly, no. You can only use Zack Fair.
When you defeat Sephiroth, Tifa gives you the Fenrir Keyblade.
Sephiroth's father is Hojo, a famous scientist who is in Final Fantasy 7 and Final Fantasy 7 Crisis Core, and his mother is Lucrecia Crescent, who worked with him on the Jenova project.
No. Just No. Sephiroth is to purely awesome.
Genesis and Angeal having wings in Final Fantasy Crisis Core is never exactly explained. It is believed to be due to them being experiments of Project Jenova as they were born in a similar way to Sephiroth.
Yes.
Yes.
If Sora can, anyone can do it.
Unfortunately, I doubt it.
No.
Absolutely he would defeat him in one attack.
Defeat Sephiroth.